Alright. Thanks, nothing happens with any of the pins when I switch on the hot-end.

I'll try with another MOSFET, but ultimately I think that the connections going to/from the MOSFET might be burnt from the small fire I had during the first start up. There might be some other connections that have burnt. See here how it looks, I have some extra wires soldered on there to try to connect, but nothing makes any difference.

I don't know how to read the circuit diagram on the Sanguinololu wiki, so I don't get which thing needs to be hooked up to what. Is there a circuit diagram for dummies somewhere, only talking about this MOSFET set-up?

basically what y did is correct. and the board should work, u r saying there is a constant 12V on the heater, meaning the heater is activated or what, the heater is has a 12v supply and control by open or close ciruilt on the ground side. if your problem is heating up whether u switch it on or not then u have to recheck your soldering and the board again